That Phono input is a line level input, and may not have enough gain for a guitar. If you keep the Mic pre in the circuit Use the 470K resistor. That is where the 2 channels are mixed together. First I would plug a guitar into the phono in jack and see if it has enough gain with the vol. all the way up.I'd like to add an input back before the 6AU6 but without a volume pot following it.
I think you're on the right track there(But I'm not one of the experts here, perhaps one of the experts will chime in here)It looks like I could just put a pot in at R16 with the wiper going into the 6L6, right? Would a 500K pot do it?